Carboniferous Limestones of Llangollen

Dr. Jacqui Malpas chose the best day of the year to lead ten MGA members on a trip to Llangollen. Strata of Silurian and Carboniferous age were examined. This account selects just three localities that presented particular difficulties of interpretation. The localities are among those described by Somerville et alii, in the 1986 guide, 'Geology around the University towns: Liverpool'. Locality 3, NGR SJ230434 This locality was described as a palaeokarstic hollow in the limestone filled with mudstone and pebbles, the pebbles showing imbricate structure and cross bedding. The group found that the pebbles ranged in size up to about 50mm in the longest dimension. Most were red in colour with pale zones interpreted as reduction spots. The fine grained mudstone was colour banded, presumably for the same reason. The pebbles were tabular and sub-rounded rather than well-rounded. A test with hydrochloric acid demonstrated that the cement was not calcareous. Rather disconcertingly many of the pebbles looked more like nodules than clasts, although that may be a feature of the way they weather. A larger clast was broken and the dark purple fracture surface revealed dark fine grained minerals with a high proportion of muscovite. Superficially all the clasts appeared to have the same mineralogy and no carbonate clasts, for instance, were identified. Whatever the origin of the pebbles, there is no doubt that they now form part of the infill of some palaeokarstic feature and are therefore evidence of a significant event when relative sea level fell and exposed the limestone to subaerial erosion. There are many subaerial erosion surfaces in the Dinantian of North Wales, not many as unequivocal as this one. The mudstone and pebbles are exposed only in two dimensions, so it was not clear what the shape of the hollow was; nor was it possible to trace the top surface laterally to confirm that the top of the pebble beds coincided with the original ground surface.

Limestone with mudstone and pebble beds above

Close up of pebble beds

Locality 5, NGR SJ233433 This small quarry, with faces about 15m high, is in the Cefn Mawr Limestone Formation. Lunch was taken while we watched a man belaying in turn two very young children as each abseiled down the face. The lowest two metres of the face displayed limestone units of approximately half a metre thick separated by calcareous mudstones up to about 50mm thick. Several colonial corals were observed, all in the base of the limestone units and all upside down. It was agreed that these units were probably storm beds. That interpretation was reinforced by the finding of load casts, which also imply rapid emplacement.
Load cast from overlying limestone unit through calcareous mudstone	Unit (A) overlying massive limestone unit (B)
The top surface of one unit was puzzling ('B' in photograph). It was undulatory and was immediately overlain by a unit ('A' in photograph) that appeared to be bioturbated. The 'bioturbation' stopped abruptly at the boundary between the units. One possibility is that the lower unit was cemented prior to the emplacement of the unit that became 'bioturbated'. If so then what processes were responsible for the cementation? Another is that the undulation is hummocky cross stratification; the unit above certainly contains clasts, as can be seen in the photograph. Perhaps what appears to be bioturbation is a chaotic mixture of relatively soft rip up clasts that have deformed after emplacement. The question will have to be answered on another day.
The group examining the Minera Formation at locality 6	Close up of the Minera Formation, thickness of brown horizons about 10mm

Location 6, NGR SJ241429 Somerville described these beds as Morton's Sandy Limestone. They form part of what is now called the Minera Formation. The bedding appeared to be on a centimetre scale; however, closer inspection showed that the pale and brown horizons both consisted of well sorted quartz grains. The brown horizons are lightly calcite cemented; in the pale horizons the interstitial space is completely filled with calcite cement. Superficial ooliths are found the pale horizons but none were seen with a hand lens. This facies crops out over about 50m laterally, truncated above by a major cross set. The author of these notes has seen a facies of similar appearance at the base of the Middle Jurassic Lincolnshire Limestone Formation. This rock type, called the Collyweston facies, also consists of quartz sand strongly and weakly calcite cemented in layers; a photograph of a cut section is shown below. At Collyweston the rock was mined and split for roofing slate. Its mode of emplacement is not known.

Finally the group walked back to Llangollen along the canal tow path, pausing occasionally to see shoals of fish just below the surface and an abundance of wild flowers along the banks. Jacqui was thanked for leading us on a stimulating day out and for arranging wall-to-wall sunshine. Rarely can the skies above Llangollen have been so blue. Peter del Strother

Collyweston facies of Middle Jurassic Lincolnshire limestone formation